Serotonin is a monoaminergic neurotransmitter involved in a wide variety psychiatric disorders, particularly depression. Central serotonin neurons are the primary targets for tricyclic antidepressants and selective serotonin reuptake inhibitors (SSRIs). Tryptophan hydroxylase (TPH) is the first-step and rate-limiting enzyme in serotonin biosynthesis. The metabolism of tryptophan initiates in the liver and leads to formation of kynurenine. The concentrations of these compounds and constituents of other metabolic pathways can be analyzed through metabolomics, a new biotechnology that applies advanced mass spectrometry techniques to characterize entire metabolic pathways. The resultant "metabolic signature" can then be determined for an individual or group of individuals. Recently, our group reported an association between a specific single nucleotide polymorphism (C1463G) in the human tryptophan hydroxylase-2 gene (TPH2) and increased risk of depression, and increased risk of refractoriness to antidepressant medications. This application will examine the following specific aims: 1) determine levels of key substrates in tryptophan metabolism in depressed patients versus controls. 2) To use metabolomic technology to examine tryptophan metabolism and other metabolic pathways. 3) To gather pilot data to examine levels of key substrates in tryptophan synthesis and metabolism among depressed patients with and without C1463G polymorphisms in TPH2. We will recruit 40 depressed patients and 20 never-depressed controls. Patients will meet criteria for unipolar major depression and will be recruited from three sites to ensure broad ranges of severity and prior exposure to antidepressants: the electroconvulsive therapy program, an outpatient mood disorders clinic, and a general internal medicine clinic. Once depression diagnoses are established and other psychiatric disorders are ruled out, subjects will have blood drawn. We will aim to get 10 subjects with the TPH-2 C1463G mutation. Metabolic signatures will be statistically analyzed between patients and controls and by severity within the depressed group. Pilot data on depressed subjects with the TPH-2 C1463 mutation will also be obtained and compared with ofher depressed subjects and controls. Results from this study will help inform future treatment studies using genetics and metabolomic technology. [unreadable] [unreadable] [unreadable]